WordNet records no synonym and no antonym for black economy. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a hidden sector of the economy where private cash transactions go unreported
“no one knows how large the black economy really is”
Synonyms, antonyms and the broader and narrower relations for black economy come from Princeton University’s WordNet. Full sources and licences.